The primary goal of this proposal is to better understand the role of TLR4 in regulating the physiologic and biologic response to inhaled endotoxin (LPS). Endotoxin is important in a number of human pulmonary diseases, including organic dust induced airway disease, pneumonia, acute lung injury, and ARDS. As a result of previous work, it is known that inhaled LPS can cause and exacerbate airway disease; that macrophages, PMNs, and airway epithelia are biologically involved in this inflammatory process; and that polymorphisms in TLR4 (a transmembrane receptor for LPS) alter the biologic and physiologic response to inhaled LPS. However, the mechanism(s) and cells through which TLR4 modulates the airway response to inhaled LPS are not known, the ability of TLR4 to initiate LPS induced PMN recruitment is unexplored, and the physiologic and biologic implications of the common co-segregating mutations in TLR4 (Asp299Gly and Thr399IIe) that are associated with LPS hyporesponsiveness in humans have not yet been investigated. In this study, TLR4 -/- mice will be reconstituted with transgenes, which drive cell specific expression of human TLR4. These reagents will be phenotyped to determine which cells are essential to the biological and physiologic response to inhaled LPS. A bone marrow transplantation model will be used to investigate the importance of TLR4 expression by combinations of cells in the response to inhaled LPS. Finally, TLR4 -/- mice reconstituted with mutant human TLR4 will be phenotyped to inhaled LPS. Airway resistance will be measured by Penh and confirmed with APTI. Bronchoalveolar lavage fluid will be sampled for cellular inflammation, protein concentration, and cytokine profile. Lung tissue will be harvested for histology and mRNA expression by Taqman analysis.